The present invention relates to an arrangement for advancing sheet material in general, and more particularly to a pressing roller unit for use in such an arrangement.
It is well known that, in arrangements for advancing sheet material, such as webs, particularly paper webs or the like in the printing industry, one or a plurality of driven rollers contact the sheet material and advance the same in a predetermined path, for instance, through a plurality of printing stations or the like. The driven rollers may be arranged, for instance, at the location at which the web enters the arrangement, at a cooling arrangement or the like. Usually, a machine of the type here under consideration includes more than one of the driven rollers. Experience has shown that, because of the tension which prevails in the material of the paper web or the like, there exists the danger that slippage could occur between the driven roller and the paper web. To avoid this problem, it has been proposed in the past to provide one or a plurality of pressing rollers which press the paper web against the driven roller with a predetermined force to increase the friction between the driven roller and the paper web and thus reduce the likelihood of the occurrence of slippage between the driven roller and the paper web being advanced thereby.
So, for instance, a German Patent DT-PS No. 10 55 553 discloses an arrangement in which a pressing roller presses the paper web against a speed-controlling roller, the purpose of the provision of the pressing roller being to assure unproblematical entrainment of the paper web by the rotating speed-controlling roller. A particular disadvantage of this conventional arrangement is that the force with which the pressing roller is urged against the speed-controlling roller can be manually adjusted only within very narrow limits.
More recently, it has been proposed in a German published patent application DT-AS No. 19 19 762 published on Nov. 5, 1970 to construct the pressing roller as a rubber roller and to urge this pressing roller against the driven roller by means of pneumatically operated cylinder-and-piston units, the operation of which is controlled by a magnetically energized valve. However, even this arrangement utilizing the pneumatically operated cylinder-and-piston units still has some grave disadvantages. The relatively high friction between the piston and the cylinder of this unit distorts the loading results in such a manner that a precisely defined loading of the pressing roller is impossible. This is further aggravated by the fact that the loading of the bearings occurs in the transverse direction of the bearings of the piston rod, as a result of which there comes into existence an even additional friction. In addition thereto, this conventional arrangement requires a substantial amount of available space, which is of a particular disadvantage especially in an arrangement where several webs or loops of a single web are located in close proximity of each other.